An organic electronic element is an element which performs an electrical operation using an organic substance, and is expected to exhibit features such as energy saving, low price, and flexibility. Thus, attention is being paid to the organic electronics as a technology to be substituted for the conventional inorganic semiconductors containing silicon as a main component.
Among organic electronic elements, organic EL elements have attracted attention for their use in, for example, large-sized solid state light sources as the substitute for incandescent lamps and gas-filled lamps. Furthermore, organic EL elements are also attracting attention as the most promising self-luminescent displays that will be substituted for liquid crystal displays (LCD) in the field of flat panel displays (FPD), and productization of the organic EL elements is in progress.
Organic EL elements are broadly classified into two classes such as low molecular weight organic EL elements and high molecular weight organic EL elements, in terms of the material and film-forming method used. High molecular weight organic EL elements are such that the organic material is composed of a polymeric material, and are capable of simplified film formation such as printing or ink-jetting as compared with low molecular weight organic EL elements which require a vacuum system for film formation. Therefore, the high molecular weight organic EL elements are indispensable elements for the large screen organic EL displays of the future.
Active research has been conducted hitherto on both the low molecular weight organic EL elements and the high molecular weight EL elements, but there are still serious problems of low light emission efficiency and short element service life. In an attempt to address these problems, multilayering has been adopted in the low molecular weight organic EL elements.
An example of a multilayered organic EL element is presented in FIG. 1. In FIG. 1, a layer that is in charge of light emission is described as a light emitting layer 1, and if the organic EL element has other layers, a layer that is in contact with an anode 2 is described as a hole injection layer 3, while a layer that is in contact with the cathode 4 is described as an electron injection layer 5. Furthermore, when another layer is present between the light emitting layer 1 and the hole injection layer 3, the layer is described as a hole transport layer 6. When another layer is present between the light emitting layer 1 and the electron injection layer 5, the layer is described as an electron transport layer 7. Meanwhile, reference numeral 8 in FIG. 1 represents a substrate.
In a low molecular weight organic EL element, since film formation is carried out by a vapor deposition method, multilayer can be easily achieved by performing vapor deposition while sequentially changing the compound used. On the other hand, in a high molecular weight organic EL element, since film formation is carried out using a wet process such as printing or ink-jetting, in order to achieve multilayering, a method in which the layer that has previously been formed does not change when a new layer is formed, is needed. The reason why multilayering is difficult in high molecular weight organic EL elements is that a lower layer which has been formed earlier is dissolved out at the time of the formation of an upper layer.
In order to take measures to this problem, investigation has been conducted on the use of compounds having significantly different solubilities. As a representative example thereof, there may be mentioned an element having a two-layer structure including a hole injection layer formed from polythiophene:polystyrene sulfonic acid (PEDOT:PSS), which is formed by using an aqueous dispersion liquid, and a light emitting layer which is formed by using an aromatic organic solvent such as toluene. In this case, since the PEDOT:PSS layer does not dissolve in toluene, it is possible to produce a two-layer structure. However, it is difficult to eliminate water, and this causes deterioration of the characteristics of the organic electronic element. Furthermore, drying at a high temperature for a long time is required for the removal of water, so that production of an organic electronic element on a resin substrate becomes difficult, or restrictions are imposed on the process such as the pressure reduction conditions.
Furthermore, as an example of using an organic solvent, there has been disclosed a method of selecting a solvent that does not affect a lower layer that has been formed earlier (see Patent Document 1).
However, because the solvent that can be used in such a method is limited to a solvent which does not dissolve the lower layer, there is a problem that the selection range for the material is narrow. Further, a certain extent of erosion of the lower layer occurs at the time of the formation of an upper layer.
As another method for constructing a multilayer structure, a method of using a crosslinking reaction has been disclosed. Patent Document 2 discloses a method of crosslinking a triphenylamine-containing ether polyether ketone by irradiating the compound with ultraviolet radiation, and thereby insolubilizing the compound. In order to achieve sufficient insolubilization by this method, ultraviolet irradiation for a long time is required, and there is a problem that decomposition of triphenylamine and the like occurs.
In addition, Patent Document 3, Patent Document 4, Non-Patent Document 1 and Non-Patent Document 2 disclose multilayering as a result of the crosslinking of oxetane groups. In these methods, since a photoinitiator is used, there is a concern about deterioration by light. Furthermore, sufficient insolubilization does not proceed at a low temperature, and there occurs a problem that the application of resin substrates which require low temperature curing is restricted, or a problem that at the time of the formation of an upper layer, the upper layer and the lower layer are mixed with each other, and the organic EL characteristics are deteriorated. Moreover, the photoinitiators used in those methods are general iodonium salts or sulfonium salts, and there is a concern about the effect of the salts on the EL characteristics.
On the other hand, in order to achieve a decrease in the driving voltage, which is an unsolved problem of organic EL elements, an investigation has been conducted on the use of an iodonium salt or sulfonium salt having the a similar structure in the hole transport layer or in the light emitting layer.
Patent Document 5 discloses an ionic compound, but this has the same structure as that of the photoinitiator described above, and there is a concern about its effect on the characteristics of organic EL elements. Further, there are no descriptions on crosslinking or lamination.
Patent Document 6 discloses a polymeric illuminant composition containing a polymeric illuminant and an ion couple. According to this literature, there is a description that when the composition has an ion couple having a particular structure, a luminescent element having a much longer service life may be obtained; however, there are no descriptions on the injection and transport of charges. Further, there are no descriptions on crosslinking or lamination.
On the other hand, for the purpose of increasing the efficiency of organic EL elements, development of phosphorescent organic EL elements is also actively attempted. In the phosphorescent organic EL elements, not only singlet state energy but also triplet state energy can be utilized, and the internal quantum yield can be increased up to 100% in principle. In a phosphorescent organic EL element, phosphorescence emission is extracted by doping a metal complex-based luminescent material containing a heavy metal such as platinum or iridium as a dopant emitting phosphorescence, into a host material (see Non-Patent Document 3, Non-Patent Document 4, and Non-Patent Document 5).
Patent Document 7 disclose a phosphorescent organic EL element in which multilayering is achieved by polymerizing a polymerizable compound. In the organic EL element, since a polymerization initiator is contained in a layer that is adjacent to a light emitting layer, it is speculated that when the polymerization initiator or a decomposition product thereof reacts with the compound of the light emitting layer, the service life of the organic EL element is shortened. For this reason, a structure which does not contain a polymerization initiator in a layer that is adjacent to the light emitting layer has been suggested.